Sliding member and method for producing same

ABSTRACT

A sliding member includes: a base; a chromium-based hard chromium plated layer formed on the surface of the base; a hard carbon layer that is mainly composed of carbon element and is formed on the hard chromium plated layer. The hydrogen concentration of the hard chromium plated layer is equal to or less than 150 mass ppm. 
     A method for producing the sliding member involves heating the surface of the base on which the chromium-based hard chromium plated layer has been formed at a temperature of 250° C. or more so that the hydrogen concentration of the hard chromium plated layer is equal to or less than 150 mass ppm, and thereafter forming the hard carbon layer mainly composed of carbon element on the hard chromium plated layer.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a divisional of U.S. application Ser. No. 14/763,011filed on Jul. 23, 2015, which is the National Phase Application ofPCT/JP2013/084212, filed on Dec. 20, 2013, which claims benefit ofpriority from the prior Japanese Application No. 2013-013290, filed onJan. 28, 2013; the entire contents of all of which are incorporatedherein by reference.

TECHNICAL FIELD

The present invention relates to a sliding member and a method forproducing the same. In more detail, the present invention relates to asliding member in which a predetermined hard chromium plated layer and ahard carbon layer are laminated on the surface of a base in the writtenorder, and to a method for producing the same.

BACKGROUND ART

A piston ring for an automobile engine has been proposed, which hasremarkably good low-friction property and can improve fuel-efficiencymore than conventional combinations of an iron material and an organicmolybdenum compound (see Patent Document 1).

The piston ring for an automobile engine is used for automobile enginesthat include a sliding member sliding in the presence of lubricatingoil. The sliding surface of the sliding member is coated with a hardcarbon film that contains hydrogen atoms in an amount of 25 at % orless. In the piston ring for an automobile engine, the hard carbon filmhas improved adhesion and improved durability, which are obtained by apretreatment of chromium plating on the sliding surface of the slidingmember prior to the hard carbon film coating.

CITATION LIST Patent Literature

Patent Document 1: Japanese Patent Unexamined Publication No. 2005-2888

SUMMARY OF INVENTION Technical Problem

However, as a result of a study of the present inventors, they foundthat a further improvement must be made even in the piston ring for anautomobile engine described in Patent Document 1 with regard to thepeeling resistance when it is subjected to high-speed sliding and a highload.

The present invention was made in view of the above-described problemwith the prior art. It is an object of the present invention to providea sliding member that has good peeling resistance and a method forproducing the same.

Solution to Problem

The present inventors conducted a keen study in order to accomplish theabove-described object. As a result, they found that the above-describedobject is accomplished by providing a predetermined hard chromium platedlayer and a hard carbon layer that are laminated on the surface of abase in the written order. The present invention was thus completed.

That is, the sliding member of the present invention includes a base, achromium-based hard chromium plated layer formed on the surface of thebase, and a hard carbon layer that is mainly composed of carbon elementand is formed on the hard chromium plated layer. In the sliding memberof the present invention, the hydrogen concentration of the hardchromium plated layer is from 10 to 140 mass ppm.

The method for producing the sliding member of the present invention isone aspect of the methods for producing the sliding member of thepresent invention. In this aspect, the surface of the base, on which thechromium-based hard chromium plated layer has been formed, is heated ata temperature of from 260° C. to less than 400° C. so that the hydrogenconcentration of the hard chromium plated layer is from 10 to 140 massppm. Thereafter, the hard carbon layer mainly composed of carbon elementis formed on the hard chromium plated layer.

Advantageous Effects of Invention

According to the present invention, the sliding member includes: thebase; the chromium-based hard chromium plated layer formed on thesurface of the base; and the hard carbon layer that is mainly composedof carbon element and is formed on the hard chromium plated layer,wherein the hydrogen concentration of the hard chromium plated layer isfrom 10 to 140 mass ppm. This configuration allows providing the slidingmember with good peeling resistance and the method for producing thesame.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is an explanatory view of a sliding member according to anembodiment of the present invention, illustrating the outline thereof.

FIG. 2 is a graph illustrating the relationship between heatingtemperature and hardness or abrasion loss of a chromium plated layer.

FIG. 3 are (a) a plan view and (b) a partial cross-sectional view of apiston ring, which is an example of a sliding member according to anembodiment of the present invention.

FIG. 4 are (a) a plan view and (b) a partial cross-sectional view of apiston ring, which is another example of a sliding member according toan embodiment of the present invention.

FIG. 5 is a scanning electron micrograph illustrating the surfacecondition of a sliding member of Example 1.

FIG. 6 is a scanning electron micrograph illustrating the surfacecondition of a sliding member of Example 2.

FIG. 7 is a scanning electron micrograph illustrating the surfacecondition of a sliding member of Comparative Example 1.

FIG. 8 is a scanning electron micrograph illustrating the surfacecondition of a sliding member of Comparative Example 3.

DESCRIPTION OF EMBODIMENTS

Hereinafter, a sliding member according to an embodiment of the presentinvention and a method for producing the sliding member will bedescribed in detail referring to the drawings. The dimension of thedrawings referred to in the following description is exaggerated fordescriptive reasons, and may be different from the actual dimension.

FIG. 1 is an explanatory view of the sliding member according to theembodiment of the present invention, illustrating the outline thereof.As illustrated in FIG. 1, the sliding member 1 of this embodimentincludes: a base 2; a chromium-based hard chromium plated layer 4 formedon the surface of the base 2; and a hard carbon layer 6 that is mainlycomposed of carbon element and is formed on the hard chromium platedlayer 4. In the present invention, the hydrogen concentration of thehard chromium plated layer 4 is equal to or less than 150 mass ppm.

Regarding a sliding member in which a hard chromium plated layer and ahard carbon layer are laminated on a base, the present inventors studiedthe peeling resistance of the hard carbon layer and found that thepeeling resistance is largely dependent on the properties of the hardchromium plated layer, which severs as an undercoat.

That is, during a process of forming a hard carbon layer on a hardchromium plated layer, the hard chromium plated layer is exposed to avacuum and/or high-temperature environment, and hydrogen present in thehard chromium plated layer is released as a gas. The released hydrogenthen inhibits carbon deposition on the hard chromium plated layer. Thepresent inventors found that this is a cause of the decreased adhesionbetween the hard chromium plated layer and the hard carbon layer.

After further advancing the study, they found that the adhesion betweenthe hard chromium plated layer and the hard carbon layer is remarkablyimproved when the surface of the base with the chromium-based hardchromium plated layer formed thereon is heated at a temperature of 250°C. or more prior to forming the hard carbon layer so that the hydrogenconcentration of the hard chromium plated layer is equal to or less than150 mass ppm.

Although the lower hydrogen concentration of the hard chromium platedlayer is better, the hard chromium plated layer contains a lot ofhydrogen at the time of formation. When the hydrogen is released by aheat treatment, a higher temperature can release more hydrogen. However,as is seen from the graph of FIG. 2, which illustrates the relationshipbetween heating temperature and hardness or abrasion loss of thechromium plated layer, when the heating temperature is equal to orgreater than 400° C., the hardness of the chromium plated layer isdecreased, and the wear resistance of the chromium plated layer isdecreased accordingly. Therefore, it is preferred that the heatingtemperature is less than 400° C.

Thank to the good peeling resistance, the sliding member of the presentinvention can be used either in the presence or absence (so-called drycondition) of a lubricant. However, since a base material (e.g. baseoil) or an additive of a lubricant can be adsorbed on the surface of thehard carbon layer to decrease the friction to further prevent peeling,it is preferred that the sliding member is used in the presence of alubricant (in a lubricant).

The sliding member may be a sliding member of a valve train, an intakeand exhaust system or a power train of internal-combustion engines suchas two- and four-stroke engines, in particular automobile engines.However, the present invention is not limited thereto but is alsoapplicable to variable swash plates or rotating vanes of refrigerantcompressors.

Examples of sliding members of a valve train or an intake and exhaustsystem include, for example, piston rings, piston pins, pistons (orpiston skirts (a piston skirt means a skirt portion of a piston),cylinders (or cylinder liners), plungers, check valves, valve guides,connection rods, bushes, crank shafts, cum lobes, cum journals, lockerarms, valve springs, shims, lifters, rotating vanes of vane pumps,housings of vane pumps, timing chains, sprockets, chain guides (or chainguide shoes), chain tensioners (or chain tensioner shoes) and the like.

Examples of sliding members of a power drive include, for example,gears, chains, belts, roller bearings, slide bearings, oil pumps etc. ofautomatic transmissions, continuously variable transmissions, manualtransmissions, final drive gears, and the like.

Bases that can be used in the present invention include metal memberssuch as high-purity iron members, aluminum members and titanium members.In addition, iron alloy members (e.g. stainless steel (steel)), copperalloy members, aluminum alloy members, magnesium alloy members andtitanium alloy members may also be used. Furthermore, non-metal memberssuch as resin members like rubber or plastic, ceramic members and carbonmembers may also be used. In particular, iron alloy members, aluminumalloy members and magnesium alloy members are preferred since they arereadily applicable to sliding members of existing machines andapparatuses. Also, they can contribute to energy saving widely invarious fields. Furthermore, it is also advantageous that such metal ornon-metal members are coated with a variety of films. For example, ironalloy members, aluminum alloy member, magnesium alloy members, titaniumalloy members and the like may be coated with a film of titanium nitride(TiN), chromium nitride (CrN) or the like.

Preferred iron alloy members that can be used include, for example,iron-based alloys containing nickel, copper, zinc, chromium, cobalt,molybdenum, lead, silicon, titanium or any combination thereof. Forexample, high-carbon chromium bearing steels (defined as SUJ2 in JISG4805), alloy tool steels, blister steels, low-alloy chilled castedirons, heat-treated carbon steels, hardened steels and the like can beused. Specific examples include nickel-chromium steels (SNC415, SNC815),nickel-chromium-molybdenum steels (SNCM220, SNCM415, SNCM420, SNCM616,SNCM815), chromium steels (SCr415, SCr420), chromium-molybdenum steels(SCM415, SCM418, SCM420, SCM421, SCM822), manganese steels (SMn420),manganese-chromium steels (SMnC420) and the like. However, the ironalloy members are not limited thereto.

The surface hardness of such iron members and iron alloy members arepreferably from HRC 45 to HRC 60 in Rockwell hardness (C scale). Suchhardness is advantageous because the base can maintain the durability ofthe hard carbon layer even in a sliding condition of a high surfacepressure of approximately 700 MPa, to which cum follower members areexposed for example. When the surface hardness is less than HRC 45,bucking and peeling may be more likely to be caused.

Further, in terms of the stability of sliding, it is preferred that thesurface roughness of such iron members and iron alloy members is equalto or less than 0.1 μm in arithmetic average roughness Ra. If thesurface roughness is greater than 0.1 μm, a local scuffing may beformed, causing a large increase in friction coefficient.

Preferred aluminum alloy members that can be used include, for example,hypoeutectic aluminum alloys or hypereutectic aluminum alloys containingfrom 4 mass % to 20 mass % of silicon (Si) and from 1.0 mass % to 5.0mass % of copper (Cu). Specific examples include AC2A, AC8A, ADC12 andADC14 defined in JIS and the like.

The surface hardness of such aluminum members and aluminum alloy membersis preferably from HB 80 to HB 130 in Brinell hardness. If the surfacehardness of aluminum members and aluminum alloy members is out of theabove-described range, i.e. if it is less than HB 80, the aluminummembers and aluminum alloy members may wear out more easily.

Further, the surface roughness of such aluminum members and aluminumalloy members are preferably equal to or less than 0.1 μm in arithmeticaverage roughness Ra in terms of the stability of sliding. If thesurface roughness is greater than 0.1 μm, a local scuffing may beformed, causing a large increase in friction coefficient.

Preferred magnesium alloys that can be used include, for example,magnesium-aluminum-zinc (Mg—Al—Zn) alloys, magnesium-aluminum-rare earthmetal (Mg—Al-REM) alloys, magnesium-aluminum-calcium (Mg—Al—Ca) alloys,magnesium-zinc-aluminum-calcium (Mg—Zn—Al—Ca) alloys,magnesium-aluminum-calcium-rare earth metal (Mg—Al—Ca-REM) alloys,magnesium-aluminum-strontium (Mg—Al—Sr) alloys,magnesium-aluminum-silicon (Mg—Al—Si) alloys, magnesium-rare earthmetal-zinc (Mg-REM-Zn) alloys, magnesium-silver-rare earth metal(Mg—Ag-REM) alloys and magnesium-yttrium-rare earth metal (Mg—Y-REM)alloys, and any combination thereof. Specific examples are AZ91, AE42,AX51, AXJ, ZAX85, AXE522, AJ52, AS21, QE22 and WE43 defined in ASTM, andthe like.

The surface hardness of such magnesium members and magnesium alloymembers is preferably from HB 45 to HB 95 in Brinell hardness. If thesurface hardness of such magnesium members and magnesium alloy membersis out of the above-described range, i.e. if it is less than HB 45, thealuminum members or aluminum alloy members may wear out more easily.

Further, in terms of the stability of sliding, it is preferred that thesurface roughness of such magnesium members and magnesium alloy membersis equal to or less than 0.1 μm in arithmetic average roughness Ra. Ifthe surface roughness is greater than 0.1 μm, a local scuffing may beformed, causing a large increase in friction coefficient.

The hard chromium plated layer of the present invention may be achromium-based plated layer. As used herein, the term “chromium-based”means that the chromium content in a plated layer is equal to or greaterthan 50 mass %. The composition of the hard chromium plated layer istypically from 0.03 mass % to 0.1 mass % of hydrogen, from 0.2 mass % to0.5 mass % of oxygen and the remaining part of chromium, which mayslightly change depending on the type of a plating bath orelectrodeposition conditions though. In the sliding member according tothe present invention, the hydrogen concentration of the hard chromiumplated layer is equal to or less than 150 mass ppm, to preferably from10 mass ppm to 140 mass ppm, more preferably from 25 mass ppm to 110mass ppm. When the hydrogen concentration is from 10 mass ppm to 140mass ppm, it is preferred that the heating temperature is from 260° C.to less than 400° C. When the hydrogen concentration is from 25 mass ppmto 110 mass ppm, it is preferred that the heating temperature is from290° C. to 360° C.

Hard carbon layers formed by chemical vapor deposition (CVD) or physicalvapor deposition (PVD), which are described below, have high owninternal stress compared to other surface treatments such as plating andthereby have remarkably high hardness. Accordingly, when a hard carbonlayer is used in a sliding member of a mechanical component, it may peeloff from the base or crack. However, the hard chromium plated layer,which is formed as an undercoat, can decrease the internal stress andthereby can make an improvement while maintaining the adhesion betweenthe hard carbon layer and the base.

The surface hardness of the hard chromium plated layer is typicallyapproximately from 800 to 1000 HV, preferably equal to or more than 700HV, more preferably equal to or more than 750 HV, yet more preferablyequal to or more than 800 HV in 10 g-load micro Vickers hardness.

Further, the thickness of the hard chromium plated layer is preferablyfrom 0.05 to 200 μm, more preferably from 0.3 to 100 μm. When thethickness is less than 0.05 μm, the friction reducing effect may be lostin a while due to initial wear. Adversely, when the thickness is greaterthan 200 μm, the layer may peel off from the base due to the increasedresidual interlayer stress.

Hard carbon layers that can be used in the present invention includecrystalline and non-crystalline layers mainly composed of carbonelement. Such crystalline layers include layers of diamond. Suchnon-crystalline layers include layers having a carbon-carbon bondinggeometry of both diamond structure (SP³ bond) and graphite structure(SP² bond), layers mainly having the graphite structure (SP² bond),layers of so-called diamond-like carbon (DLC), and the like. Specificexamples of the non-crystalline layers having a carbon-carbon bondinggeometry of both diamond structure (SP³ bond) and graphite structure(SP² bond) include amorphous carbon (a-C) layers consisting of carbonelement (containing no hydrogen), hydrogen-amorphous carbon (a-C:H)layers containing hydrogen, metal amorphous carbon (MeC) layerscontaining a metal element such as titanium (Ti) or molybdenum (Mo) as acomponent, and the like. Further, tetrahedral amorphous carbon (ta-C)layers, which are non-crystalline layers having a carbon-carbon bondinggeometry of mainly the diamond structure (SP³ bond), can be also used.

The hard carbon layer is formed by CVD, PVD or the like. In general,hard carbon layers formed by CVD such as thermal CVD and plasma CVDcontain hydrogen derived from an organic compound material (e.g.hydrocarbon gas). The hydrogen concentration in such hard carbon layersis typically from 15 at % to 40 at %. In contrast, inclusion orexclusion of hydrogen can be controlled in PVD such as ion plating usinga carbon beam, arc ion plating, laser abrasion, sputtering and magnetronsputtering. The friction is reduced more as the hydrogen concentrationof the hard carbon layer is lower. Accordingly, the hydrogenconcentration of the hard carbon layer is preferably equal to or lessthan 40 at %, more preferably equal to or less than 25 at %, morepreferably equal to or less than 10 at %, more preferably equal to orless than 5 at %, more preferably equal to or less than 2 at %, morepreferably equal to or less than 0.3 at %, more preferably equal to orless than 0.1 at %. In particular, hydrogen amorphous carbon (a-C:H)containing hydrogen in an amount greater than 0 at % and less than orequal to 1 at %, amorphous carbon (a-C) containing no hydrogen andtetrahedral amorphous carbon (ta-C) containing no hydrogen arepreferred. In terms of reducing hydrogen particularly in the outermostlayer of the hard carbon layer, the hard carbon layer may have amultilayer structure composed of two or more layers, in which theoutermost layer is made of hydrogen amorphous carbon (a-C:H) containinghydrogen in an amount greater than 0 at % and less than or equal to 1 at% or amorphous carbon (a-C) containing no hydrogen. As used herein, theterm “outermost layer” represents the 5% from the outer surface of thehard carbon layer with respect to the total thickness thereof, typicallythe range from the outer surface to 1.0 μm in depth of the hard carbonlayer. When the hard carbon layer has monolayer structure, the monolayeris referred to as the outermost layer.

Further, the surface roughness of the hard carbon layer is preferablyequal to or less than 0.1 μm in arithmetic average roughness Ra, morepreferably equal to or less than 0.08 μm, yet more preferably equal toor less than 0.05 μm, particularly equal to or less than 0.03 μm. Whenthe surface roughness is greater than 0.1 μm in arithmetic averageroughness Ra, local scuffing may be formed, causing an increase infriction coefficient. Since the smoother surface is better, the lowerlimit of the roughness is not particularly defined. However, inpractice, the surface is finished to the above-described adequateroughness in consideration of the production cost. In terms of thestability of sliding, a suitable surface roughness is equal to or lessthan 0.08 μm in arithmetic average roughness Ra.

Next, lubricants used for the sliding member of the present inventionwill be described in detail.

Such lubricants include oxygen-containing organic compounds. Theoxygen-containing organic compounds are not particularly limited and maybe any organic compound that contains oxygen in the molecule. Forexample, such oxygen-containing organic compounds may be composed ofcarbon, hydrogen and oxygen, or may contain, in addition thereto, otherelements such as nitrogen, sulfur, halogen (fluorine, chlorine, etc.),phosphor, boron and metal. In terms of further reducing the friction ofthe sliding surface between the hard carbon layer of the sliding memberand the counterpart sliding member of an arbitrary material, suitablelubricants are oxygen-containing organic compounds that are composed ofcarbon, hydrogen and oxygen and have a hydroxyl group, and thederivatives thereof. It is more preferred that these compounds have twoor more hydroxyl groups.

For the same reason, it is preferred that oxygen-containing organiccompounds have low sulfur content or contain no sulfur.

As used herein, the term “derivative” typically refers to compounds thatare obtained by reacting an oxygen-containing organic compoundcontaining carbon, hydrogen and oxygen with, for example, anitrogen-containing compound, a phosphor-containing compound, sulfur ora sulfur-containing compound, a boron-containing compound, halogen or ahalogen-containing compound, metal or a metal-containing compound, orthe like (regardless of an organic or inorganic compound), but is notparticularly limited.

Specific examples of the oxygen-containing organic compounds include,for example, compounds having a hydroxyl group, a carboxyl group or acarbonyl group, compounds having an ester bond or an ether bond (thesecompounds may have two or more groups or bonds), and the like. It ispreferred that the oxygen-containing organic compounds have one or moregroup or bond selected from hydroxyl, carboxyl, carbonyl and ester, morepreferably one or more group or bond selected from hydroxyl, carboxyland ester, yet more preferably one or more group selected from hydroxyland carboxyl, particularly one or more hydroxyl group.

More specific examples include (1) alcohols, (2) carboxylic acids, (3)esters, (4) ethers, (5) ketones, (6) aldehydes and (7) carbonates (theymay further have one or more group or bond selected from hydroxyl,carboxyl, carbonyl, ester and ether) and the derivatives thereof, andany combination thereof.

The alcohols (1) are oxygen-containing organic compounds of thefollowing formula (I).R—(OH)_(n)  (I)

For example, the alcohols (1) are compounds having one or more hydroxylgroup.

Specific examples of the alcohols (1) include the following compounds:

-   -   monohydric alcohols (1-1),    -   dihydric alcohols (1-2),    -   tri- or more hydric alcohols (1-3),    -   alkylene oxide adducts of the above three alcohols (1-4), and    -   mixtures of one or more compound selected from the above four        alcohols (1-5).

The monohydric alcohols (1-1), which have one hydroxyl group in themolecule, include, for example, monohydric alkyl alcohols of 1 to 40carbons (the alkyl may be straight or branched) such as methanol,ethanol, propanol (1-propanol, 2-propanol), butanol (1-butanol,2-butanol, 2-methyl-1-propanol, 2-methyl-2-propanol), pentanol(1-pentanol, 2-pentanol, 3-pentanol, 2-methyl-1-butanol,3-methyl-1-butanol, 3-methyl-2-butanol, 2-methyl-2-butanol,2,2-dimethyl-1-propanol), hexanol (1-hexanol, 2-hexanol, 3-hexanol,2-methyl-1-pentanol, 2-methyl-2-pentanol, 2-methyl-3-pentanol,3-methyl-1-pentanol, 3-methyl-2-pentanol, 3-methyl-3-pentanol,4-methyl-1-pentanol, 4-methyl-2-pentanol, 2,3-dimethyl-1-butanol,2,3-dimethyl-2-butanol, 3,3-dimethyl-1-butanol, 3,3-dimethyl-2-butanol,2-ethyl-1-butanol, 2,2-dimethylbutanol), heptanol (1-heptanol,2-heptanol, 3-heptanol, 2-methyl-1-hexanol, 2-methyl-2-hexanol,2-methyl-3-hexanol, 5-methyl-2-hexanol, 3-ethyl-3-pentanol,2,2-dimethyl-3-pentanol, 2,3-dimethyl-3-pentanol,2,4-dimethyl-3-pentanol, 4,4-dimethyl-2-pentanol, 3-methyl-1-hexanol,4-methyl-1-hexanol, 5-methyl-1-hexanol, 2-ethylpentanol), octanol(1-octanol, 2-octanol, 3-octanol, 4-methyl-3-heptanol,6-methyl-2-heptanol, 2-ethyl-1-hexanol, 2-propyl-1-pentanol,2,4,4-trimethyl-1-pentanol, 3,5-dimethyl-1-hexanol, 2-methyl-1-heptanol,2,2-dimethyl-1-hexanol), nonanol (1-nonanol, 2-nonanol,3,5,5-trimethyl-1-hexanol, 2,6-dimethyl-4-heptanol,3-ethyl-2,2-dimethyl-3-pentanol, 5-methyloctanol, etc.), decanol(1-decanol, 2-decanol, 4-decanol, 3,7-dimethyl-1-octanol,2,4,6-trimethylheptanol, etc.), undecanol, dodecanol, tridecanol,tetradecanol, pentadecanol, hexadecanol, heptadecanol, octadecanol(stearyl alcohol, etc.), nonadecanol, eicosanol, heneicosanol,tricosanol and tetracosanol; monohydric alkenyl alcohols (the alkenylmay be straight or branched, and the double bond may be at any position)of 2 to 40 carbons such as ethenol, propenol, butenol, hexenol, octenol,decenol, dodecenol and octadecenol (oleyl alcohol, etc.); monohydric(alkyl) cycloalkyl alcohols of 3 to 40 carbons (the alkyl may bestraight or branched, and the alkyl or hydroxyl substituent may be atany position) such as cyclopentanol, cyclohexanol, cycloheptanol,cyclooctanol, methylcyclopentanol, methylcyclohexanol,dimethylcyclohexanol, ethylcyclohexanol, propylcyclohexanol,butylcyclohexanol, dimethylcyclohexanol, cyclopentylmethanol,cyclohexylmethanol, cyclohexylethanol (1-cyclohexylethanol,2-cyclohexylethanol, etc.), cyclohexylpropanol (3-cyclohexylpropanol,etc.), cyclohexylbutanol (4-cyclohexylbutanol, etc.), butylcyclohexanoland 3,3,5,5-tetramethylcyclohexanol; (alkyl)aryl alcohols (the alkyl maybe straight or branched, and the alkyl or hydroxyl substituent may be atany position) such as phenylalcohol, methylphenylalcohol (o-cresol,m-cresol, p-cresol), creosol, ethylphenylalcohol, propylphenylalcohol,butylphenylalcohol, butylmethylphenylalcohol(3-methyl-6-tert-butylphenylalcohol, etc.), dimethylphenylalcohol,diethylphenylalcohol,dibutylphenylalcohol(2,6-di-tert-butylphenylalcohol,2,4-di-tert-butylphenyl alcohol, etc.), dibutylmethylphenylalcohol(2,6-di-tert-butyl-4-methylphenylalcohol, etc.),dibutylethylphenylalcohol (2,6-di-tert-butyl-4-ethylphenylalcohol,etc.), tributylphenylalcohol (2,4,6-tri-tert-butylphenylalcohol, etc.),naphthol (α-naphthol, β-naphthol, etc.) and dibutylnaphthol(2,4-di-tert-butyl-α-naphthol, etc.);6-(4-oxy-3,5-di-tert-butyl-anilino)-2,4-bis-(n-octyl-thio)-1,3,5-triazine;the mixtures thereof; and the like.

Among these monohydric alcohols, straight or branched alkyl or alkenylalcohols of 12 to 18 carbons such as oleylalcohol and stearylalcohol arepreferably used since they can further reduce the friction of thesliding surface between the hard carbon layer of the sliding member anda counterpart sliding member of an arbitrary material and also has sucha low volatility that they can reduce the friction even in ahigh-temperature condition (e.g. a sliding condition of an internalcombustion engine).

The dihydric alcohols (1-2), which have two hydroxyl groups in themolecule, include, for example, alkyl- or alkenyldiols of 2 to 40carbons (the alkyl or alkenyl moiety may be straight or branched, thedouble bond of the alkenyl moiety may be at any position, and thehydroxyl substituents may be at any position) such as ethylene glycol,diethylene glycol, polyethylene glycol, propylene glycol, dipropyleneglycol, polypropylene glycol, neopentyl glycol, 1,3-propanediol,1,4-butanediol, 1,2-butanediol, 2-methyl-1,3-propanediol,1,5-pentanediol, 1,6-hexanediol, 2-ethyl-2-methyl-1,3-propanediol,2-methyl-2,4-pentanediol, 1,7-heptanediol,2-methyl-2-propyl-1,3-propanediol, 2,2-diethyl-1,3-propanediol,1,8-octanediol, 1,9-nonanediol, 2-butyl-2-ethyl-1,3-propanediol,1,10-decanediol, 1,11-undecanediol, 1,12-dodecanediol,1,13-tridecanediol, 1,14-tetradecanediol, 1,15-pentadecanediol,1,16-hexadecanediol, 1,17-heptadecanediol, 1,18-octadecanediol,1,19-nonadecanediol, and 1,20-icosadecanediol; (alkyl) cycloalkanediols(the alkyl moiety may be straight or branched, and the alkyl or hydroxylsubstituents may be at any position) such as cyclohexanediol andmethylcyclohexanediol; dihydric (alkyl)aryl alcohols of 2 to 40 carbons(the alkyl moiety may be straight or branched, and the alkyl or hydroxylsubstituents may be at any position) such as benzenediol (catechol,etc.), methylbenzenediol, ethylbenzenediol, butylbenzenediol(p-tert-buthylcatechol, etc.), dibutylbenzenediol(4,6-di-tert-butyl-resorcin, etc.),4,4′-thiobis-(3-methyl-6-tert-butyl-phenol),4,4′-butylidenebis-(3-methyl-6-tert-butyl-phenol),2,2′-methylenebis-(4-methyl-6-tert-butyl-phenol),2,2′-thiobis-(4,6-di-tert-butyl-resorcin),2,2′-methylenebis(4-ethyl-6-tert-butyl-phenol),4,4′-methylenebis-(2,6-di-tert-butyl-phenol),2,2′-(3,5-di-tert-butyl-4-hydroxy)propane, and4,4′-cyclohexilidenebis-(2,6-di-tert-butyl-phenol); a condensate ofp-tert-butylphenol and formaldehyde, a condensate of p-tert-butylphenoland acetaldehyde; the mixtures thereof; and the like.

Among these dihydric alcohols, ethylene glycol, propylene glycol,neopentyl glycol, 1,4-butanediol, 1,5-pentanediol, neopentyl glycol,1,6-hexanediol, 2-methyl-2,4-pentanediol,2-ethyl-2-methyl-1,3-propanediol, 1,7-heptanediol, 1,8-octanediol,1,9-nonanediol, 1,10-decanediol, 1,11-undecanediol, 1,12-dodecanediol,and the like are preferably used since they can further reduce thefriction of the sliding surface between the hard carbon layer of thesliding member and a counterpart sliding member of an arbitrarymaterial. Further, hindered alcohols having a high molecular weight of300 or more, preferably 400 or more, such as2,6-di-tert-butyl-4-(3,5-di-tert-butyl-4-hydroxy-benzyl)phenylalcohol,are preferred since they can reduce the friction even in ahigh-temperature condition (e.g. the sliding condition of internalcombustion engines) due to its low volatility and high heat resistancein such conditions and can also impart good oxidation stability.

The tri- or more hydric alcohols (1-3), which have three or morehydroxyl groups, are polyhydric alcohols having typically 3 to 10hydroxyl groups, preferably 3 to 6 hydroxyl groups. Specific examplesinclude trimethylol alkanes such as glycerin, trimethylol ethane,trimethylol propane, and trimethylol butane; erythritol,pentaerythritol, 1,2,4-butanetriol, 1,3,5-pentanetriol,1,2,6-hexanetriol, 1,2,3,4-butanetetrol, sorbitol, adonitol, arabitol,xylitol, mannitol and the like and the polymers and condensates thereof(e.g. dimer to octamer of glycerin such as diglycerin, triglycerin andtetraglycerin, dimer to octamer of trimethylol propane such asditrimethylol propane, dimer to tetramer of pentaerythritol such asdipentaerythritol, condensates (intramolecular condensates,intermolecular condensates or self-condensates) such as sorbitan andsorbitol-glycerin condensate), and the like.

Further, sugars such as xylose, arabitol, ribose, rhamnose, glucose,fructose, galactose, mannose, sorbose, cellobiose, mantose, isomaltose,trehalose and sucrose can also be used.

Among these tri- or more hydric alcohols, tri- to hexahydric alcoholssuch as glycerin, trimethylol alkanes (e.g. trimethylol ethane,trimethylol propane, trimethylol butane), pentaerythritol,1,2,4-butanetriol, 1,3,5-pentantriol, 1,2,6-hexanetriol,1,2,3,4-butanetetrol, sorbitol, sorbitan, sorbitol-glycerin condensate,adonitol, arabitol, xylitol and mannitol, the mixtures thereof, and thelike are more preferred. Glycerin, trimethylol ethane, trimethylolpropane, pentaerythritol, sorbitan and the mixtures thereof are yet morepreferred. Polyhydric alcohols that contains 20% or more, preferably 30%or more, particularly 40% or more of oxygen are particularly preferred.Polyalcohols having more than six hydroxyl groups have too highviscosity.

The alkylene oxide adducts (1-4) are alkylene oxide adducts of theabove-described alcohols (1-1 to 1-3). Specifically, an alkylene oxideof 2 to 6 carbons, preferably 2 to 4 carbons, or a polymer or copolymerthereof is added to the alcohols so that the hydroxyl group thereof isconverted to a hydrocarbylether or a hydrocarbylester. Such alkyleneoxides of 2 to 6 carbons include ethylene oxide, propylene oxide,1,2-epoxybutane (α-butyleneoxide), 2,3-epoxybutane (β-butyleneoxide),1,2-epoxy-1-methylpropane, 1,2-epoxyheptane, 1,2-epoxyhexane and thelike. Among them, ethylene oxide, propylene oxide, butylene oxide arepreferred in terms of good low-friction property. Ethylene oxide andpropylene oxide are more preferred.

When two or more alkylene oxides are used, polymerization between theoxyalkylene groups may be of any type, including random copolymerizationand block copolymerization. Regarding the addition of an alkylene oxideto a polyhydric alcohol having two to six hydroxyl groups, the alkyleneoxide may be added to either all of or part of the hydroxyl groups.

The carboxylic acids (2) are oxygen-containing organic compounds of thefollowing formula (II).R—(COOH)_(n)  (II)

For example, the carboxylic acids (2) are compounds having one or morecarboxyl groups.

Specific examples of such carboxylic acids (2) include the followingcompounds:

-   -   aliphatic monocarboxylic acids (fatty acids) (2-1),    -   aliphatic polycarboxylic acids (2-2),    -   carbocyclic carboxylic acids (2-3),    -   heterocyclic carboxylic acids (2-4), and    -   mixtures of two or more compounds selected from the above four        carboxylic acids (2-5)

Specific examples of the aliphatic monocarboxylic acids (fatty acids)(2-1), which are aliphatic monocarboxylic acids having one carboxylgroup in the molecule, include, for example, saturated aliphaticmonocarboxylic acids of 1 to 40 carbons (the saturated aliphatic moietymay be either straight or branched) such as methanoic acid, ethanoicacid (acetic acid), propaonic acid (propionic acid), butenoic acid(butyric acid, isobutyric acid, etc.), pentanoic acid (valeric acid,isovaleric acid, pivalic acid, etc.), hexanoic acid (caproric acid,etc.), heptanoic acid, octanoic acid (caprylic acid, etc.), nonanoicacid (pelargonic acid, etc.), decanoic acid, undecanoic acid, dodecanoicacid (lauric acid, etc.), tridecenoic acid, tetradecenoic acid (myristicacid, etc.), pentadecanoic acid, hexadecanoic acid (palmitic acid),heptadecanoic acid, octadecanoic acid (stearic acid), nonadecanoic acid,icosanoic acid, henicosanoic acid, docosanoic acid, tricosanoic acid,tetracosanoic acid, pentacosanoic acid, hexacosanoic acid, heptacosanoicacid, octacosanoic acid, nonacosanoic acid and triacontanoic acid;unsaturated aliphatic monocarboxylic acid of 1 to 40 carbons (theunsaturated aliphatic moiety may be either straight or branched, and theunsaturated bond may be at any position) such as propenoic acid (acrylicacid, etc.), propinoic acid (propiolic acid, etc.), butenoic acid(methacrylic acid, crotonic acid, isocrotonic acid, etc.), pentenoicacid, hexenoic acid, heptenoic acid, octenoic acid, nonenoic acid,decenoic acid, undecenoic acid, dodecenoic acid, tridecenoic acid,tetradecenoic acid, pentadecanoic acid, hex adecenoic acid,heptadecenoic acid, octadecenoic acid (oleic acid, etc.), nonadecenoicacid, icosenoic acid, heneicosenoic acid, docosenoic acid, tricosenoicacid, tetracosenoic acid, pentacosenoic acid, hexacosenoic acid,heptacosenoic acid, octacosenoic acid, nonacosenoic acid andtriacontenoic acid; and the like.

Examples of the aliphatic polycarboxylic acids (2-2) include saturatedor unsaturated aliphatic dicarboxylic acids of 2 to 40 carbons (thesaturated or unsaturated aliphatic moiety may be either straight orbranched, and the unsaturated bond may be at any position) such asethandioic acid (oxalic acid, etc.), propanedionic acid (malonic acid,etc.), butanedioic acid (succinic acid, methylmalonic acid, etc.),pentanedioic acid (glutaric acid, ethylmalonic acid, etc.), hexanedioicacid (adipic acid, etc.), heptanedioic acid (pimelic acid, etc.),octanedioic acid (suberic acid, etc.), nonanedioic acid (azelaic acid,etc.), decanedioic acid (sebacic acid), propenedioic acid, butenedioicacid (maleic acid, fumaric acid, etc.), pentenedioic acid (citraconicacid, mesaconic acid, etc.), hexenedioic acid, heptenedioic acid,octenedioic acid, nonenedioic acid and decenedioic acid; saturated orunsaturated aliphatic tricarboxylic acids (the saturated or unsaturatedaliphatic moiety may be either straight or branched, and the unsaturatedbond may be at any position) such as propanetricarboxylic acid,butanetricarboxylic acid, pentanetricarboxylic acid, hexanetricarboxylicacid, heptanetricarboxylic acid, octanetricarboxylic acid,nonanetricarboxylic acid and decanetricarboxylic acid; saturated orunsaturated aliphatic tetracarboxylic acids (the saturated orunsaturated aliphatic moiety may be either straight or branched, and theunsaturated bond may be at any position); and the like.

Specific examples of the carbocyclic carboxylic acids (2-3), which arecarboxylic acids having one or more carboxyl group on a carbon ring inthe molecule, include, for example, mono-, di-, tri- or tetracarboxylicacids of 3 to 40 carbons having a naphthene ring (the alkyl or alkenylsubstituent (if any) may be either straight or branched, the double bondmay be at any position, and the number and position of the substituentare not limited) such as cyclohexanemonocarboxylic acid,methylcyclohexanemonocarboxylic acid, ethylcyclohexanemonocarboxylicacid, propylcyclohexanemonocarboxylic acid,butylcyclohexanemonocarboxylic acid, pentylcyclohexanemonocarboxylicacid, hexylcyclohexanemonocarboxylic acid,heptylcyclohexanemonocarboxylic acid, octylcyclohexanemonocarboxylicacid, cycloheptanemonocarboxylic acid, cyclooctanemonocarboxylic acidand trimethylcyclopentanedicarboxylic acid (camphor acid, etc.);aromatic monocarboxylic acid of 7 to 40 carbons such asbenzenecarboxylic acid (benzoic acid), methylbenzenecarboxylic acid(toluic acid, etc.), ethylbenzenecarboxylic acid, propylbenzencarboxylicacid, benzenedicarboxylic acid (phthalic acid, isophthalic acid,telephthalic acid, etc.), benzenetricarboxylic acid (trimellitic acid,etc.), benzenetetracarboxylic acid (pyromellitic acid, etc.) andnaphthalenecarboxylic acid (naphthoic acid); mono-, di-, tri- ortetracarboxylic acids of 7 to 40 carbons having an aryl group (the alkylor alkenyl substituent (if any) may be either straight or branched, thedouble bond may be at any position, and the number and position of thesubstituent are not limited) such as phenylpropanoic acid, (hydroatropicacid), phenylpropenoic acid (atropic acid, cinnamic acid, etc.),salicylic acid and alkyl salicylic acids having one or more alkyl groupof 1 to 30 carbons; and the like.

Specific examples of the heterocyclic carboxylic acids (2-4), which areheterocyclic carboxylic acids having one or more carboxyl group in themolecule, include, for example, heterocyclic carboxylic acids of 5 to 40carbons such as furancarboxylic acid, thiophenecarboxylic acid andpyridinecarboxylic acid (nicotinic acid, isonicotinic acid, etc.), andthe like.

The esters (3) are oxygen-containing organic compounds of the followingformula (III).R—(COO—R′)_(n)  (III)

For example, the esters (3) are compounds having one or more ester bond.

Specific examples of the esters (3) include the following compounds:

-   -   esters of aliphatic monocarboxylic acids (fatty acids) (3-1),    -   esters of aliphatic polycarboxylic acids (3-2),    -   esters of carbocyclic carboxylic acids (3-3),    -   esters of heterocyclic carboxylic acids (3-4),    -   alkylene oxide adducts of alcohols or esters (3-5), and    -   any mixture of compounds selected from the above five esters        (3-6).        The above esters 3-1 to 3-5 may be either complete esters, in        which all of the hydroxyl or carboxyl group is esterified, or        partial esters, in which the hydroxyl or carboxyl group partly        remains.

The esters of aliphatic monocarboxylic acids (fatty acids) (3-1) areesters of one or more of the above-described aliphatic monocarboxylicacids (2-1) and one or more of the above-described mono- di-, tri- ormore hydric alcohols (1-1 to 1-3). Specific examples of such aliphaticmonocarboxylic acids include, for example, glycerin monooleate, glycerindioleate, sorbitan monooleate, sorbitan dioleate and the like. Amongthem, fatty acid esters having a straight or branched hydrocarbon groupof 6 to 30 carbons, specifically esters of fatty acids having such ahydrocarbon group and aliphatic mono- or polyhydric alcohols areparticularly preferred. Details are described below.

Examples of the esters (3-1) include, except for particularly preferredfatty acid ester type ash-free friction adjusters, aliphatic estershaving a straight or branched hydrocarbon group of 1 to 5 carbons or 31to 40 carbons, specifically esters of fatty acids having such ahydrocarbon group and aliphatic mono- or polyhydric alcohols.

Among them, esters having a kinetic viscosity at 100° C. of from 1 to100 mm²/s can be used as lubricant base oil, which are generallydistinguishable from the particularly preferred fatty acid ester typeash-free friction adjusters. Examples of such esters include, forexample, single esters and polyol esters (e.g. complex esters, etc.) oftri- or more hydric polyols, particularly tri- or more hydric polyolshaving a neopentyl structure, of 3 to 40 carbons, preferably 4 to 18carbons, particularly 4 to 12 carbons (e.g. trimethylolpropanecaprilate, trimethylolpropane pelargonate, pentaerythritol2-ethylhexanoate, pentaerythritol pelargonate, etc.) and one or moreacid selected from monocarboxylic acids of 1 to 40 carbons, preferably 4to 18 carbons, particularly 6 to 12 carbons; mixtures thereat alkyleneoxide adducts thereof; and the like. They may be either complete esters,in which all of the hydroxyl or carboxyl group is esterified, or partialesters, in which the hydroxyl or carboxyl group partly remain, but arepreferably complete esters. The hydroxyl value is typically equal to orless than 100 mgKOH/g, more preferably equal to or less than 50 mgKOH/g,particularly equal to or less than 10 mgKOH/g. Further, the kineticviscosity at 100° C. of such lubricant base oil is preferably 2 to 60mm²/s, particularly 3 to 50 mm²/s.

The esters of aliphatic polycarboxylic acids (3-2) are esters of one ormore selected from the above-described aliphatic polycarboxylic acids(2-2) and one or more selected from the above-described mono-, di-, tri-or more hydric alcohols (1-1 to 1-3), and the like. Specific examplesinclude, for example, diesters of one or more polycarboxylic acidselected from dicarboxylic acids of 2 to 40 carbons, preferably 4 to 18carbons, particularly 6 to 12 carbons and one or more alcohol selectedfrom monohydric alcohols of 4 to 40 carbons, preferably 4 to 18 carbons,particularly 6 to 14 carbons, such as dibutyl maleate, ditridecylglutarate, di-2-ethylhexyl adipate, diisodecyl adipate, ditridecyladipate and di-2-ethylhexyl sabacate; copolymers of such a diester (e.g.dibutyl maleate, etc.) and a poly α-olefin of 4 to 16 carbons or thelike; esters of an α-olefin adduct of acetic acid anhydride or the likeand an alcohol of 1 to 40 carbons; and the like. Among them, estershaving a is kinetic viscosity at 100° C. of from 1 to 100 mm²/s can beused as lubricant base oil.

Examples of the esters of carbocyclic carboxylic acids (3-3) includeesters of one or more acid selected from the above-described carbocycliccarboxylic acids (2-3) and one or more alcohol selected from theabove-described mono-, di-, tri- or more hydric alcohols (1-1 to 1-3),and the like. Specific examples include, for example, aromaticcarboxylates such as phthalates, trimellitates, pyromellitates,salicylates. Among them, esters having a kinetic viscosity at 100° C. offrom 1 to 100 mm²/s can be used as lubricant base oil.

Examples of the esters of heterocyclic carboxylic acids (3-4) includeesters of one or more acid selected from the above-describedheterocyclic carboxylic acids (2-4) and one or more alcohol selectedfrom the above-described mono-, di-, tri- or more hydric alcohols (1-1to 1-3). Among them, esters having a kinetic viscosity at 100° C. offrom 1 to 100 mm²/s can be used as lubricant base oil.

Examples of the alkylene oxide adducts of alcohols and esters (3-5)include esterified products obtained by adding an alkylene oxide to oneor more alcohol selected from the above-described mono-, di-, tri- ormore hydric alcohols (1-1 to 1-3), alkylene oxide adducts of theabove-described esters (3-1 to 3-4), and the like. Among them, adductshaving a kinetic viscosity at 100° C. of from 1 to 100 mm²/s can be usedas lubricant base oil.

The ethers (4) are oxygen-containing compounds of the following formula(IV).R—(O—R′)_(n)  (IV)

For example, the ethers (4) are compounds having one or more ether bond.

Specific examples of the ethers (4) include, for example, the followingcompounds:

-   -   saturated or unsaturated aliphatic ethers (4-1),    -   aromatic ethers (4-2),    -   cyclic ethers (4-3), and    -   mixtures of two or more ethers selected from the above-described        three ethers (4-4).

Specific examples of the saturated or unsaturated aliphatic ethers(aliphatic simple ethers) (4-1) include, for example, saturated orunsaturated aliphatic ethers of 1 to 40 carbons (the saturated orunsaturated aliphatic moiety may be either straight or branched, and theunsaturated bond may be at any position) such as dimethyl ether, diethylether, di-n-propyl ether, diisopropyl ether, dibutyl ether, diisobutylether, di-n-amyl ether, diisoamyl ether, dihexyl ether, diheptyl ether,dioctyl ether, dinonyl ether, didecyl ether, diundecyl ether, didodecylether, ditridecyl ether, ditetradecyl ether, dipentadecyl ether,dihexadecyl ether, diheptadecyl ether, dioctadecyl ether, dinonadecylether, diicosyl ether, methylethyl ether, methyl-n-propyl ether,methylisopropyl ether, methylisobutyl ether, methyl-tert-butyl ether,methyl-n-amyl ether, methylisoamyl ether, ethyl-n-propyl ether,ethylisopropyl ether, ethylisobutyl ether, ethyl-tert-butyl ether,ethyl-n-amyl ether, ethylisoamyl ether, divinyl ether, diallyl ether,methylvinyl ether, methylallyl ether, ethylvinyl ether and ethylallylether.

Specific examples of the aromatic ethers (4-2) include, for example,anisole, phenetole, phenyl ether, benzyl ether, phenylbenzyl ether,α-naphthyl ether, β-naphthyl ether, polyphenyl ether, perfluoro etherand the like. They may have a saturated or unsaturated aliphatic group(the saturated or unsaturated aliphatic group may be either straight orbranched, the unsaturated bond may be at any position, and the positionand number of the substituent are not limited). They are preferably inthe form of liquid in their conditions in use, particularly at ordinarytemperature.

Specific examples of the cyclic ethers (4-3) include, for example,cyclic ethers of 2 to 40 carbons such as ethylene oxide, propyleneoxide, trimethylene oxide, tetrahydropyran, tetrahydropyran, dioxane andglycidyl ether. They may have a saturated or unsaturated aliphaticgroup, a carbocyclic group or a carbocyclic group having a saturated orunsaturated aliphatic group (the saturated or unsaturated aliphaticgroup may be either straight or branched, the unsaturated bond may be atany position, and the position and number of the substituent are notlimited).

The ketones (5) are oxygen-containing organic compounds of the followingformula (V).R—(CO—R′)_(n)  (V)

For example, the ketones (5) are compounds having one or more carbonylbond.

Specific examples of the ketones (5) include, for example, the followingcompounds:

-   -   saturated or unsaturated aliphatic ketones (5-1),    -   carbocyclic ketones (5-2),    -   heterocyclic ketones (5-3),    -   ketone alcohols (5-4),    -   ketone acids (5-5), and    -   mixtures of two or more ketones selected from the above five        ketones (5-6).

Specific examples of the saturated or unsaturated aliphatic ketones(5-1) include, for example, saturated or unsaturated aliphatic ketonesof 1 to 40 carbons (the saturated or unsaturated aliphatic moiety may beeither straight or branched, and the unsaturated bond may be at anyposition) such as acetone, methylethyl ketone, methylpropyl ketone,methylisopropyl ketone, methylbutyl ketone, methylisobutyl ketone,pinacolone, diethyl ketone, butylone, diisopropyl ketone, methylvinylketone, mesityl oxide and methylheptenone; and the like.

Specific examples of the carbocyclic ketones (5-2) include, for example,carbocyclic ketones of 1 to 40 carbons such as cyclobutanone,cyclopentanone, cyclohexanone, acetophenone, propiophenone,butyrophenone, valerophenone, benzophenone, dibenzyl ketone and2-acetonaphthone. They may have a saturated or unsaturated aliphaticgroup (the saturated or unsaturated aliphatic group may be eitherstraight or branched, the unsaturated bond may be at any position, andthe position and number of the substituent are not limited.).

Specific examples of the heterocyclic ketones (5-3) include, forexample, heterocyclic ketones of 1 to 40 carbons such as acetothienoneand 2-acetofurone. They may have a saturated or unsaturated aliphaticgroup (the saturated or unsaturated aliphatic group may be eitherstraight or branched, the unsaturated bond may be at any position, andthe position and number of the substituent are not limited).

Specific examples of the ketone alcohols (ketols) (5-4) include, forexample, ketone alcohols of 1 to 40 carbons such as acetol, acetoin,acetoethylalcohol, diacetone alcohol, phenacyl alcohol and benzoin. Theymay have a carbocyclic or heterocyclic ring, or a carbocyclic orheterocyclic ring having a saturated or unsaturated aliphatic group (thesaturated or unsaturated aliphatic group may be either straight orbranched, the unsaturated bond may be at any position, and the positionand number of the substituent are not limited.).

Specific examples of the ketone acids (5-5) include, for example, ketoneacids of 1 to 40 carbons such as α-ketone acids (pyruvic acid,benzoylformic acid, phenylpyruvic acid, etc.), β-ketone acids(acetoacetic acid, propionylacetic acid, benzoylacetic acid, etc.) andγ-ketone acids (e.g. levulinic acid, β-benzoylpropionic acid, etc.).

The aldehydes (6) are oxygen-containing organic compounds of thefollowing formula (VI).R—(CHO)_(n)  (VI)

For example, the aldehydes (6) are compounds having one or more aldehydegroup.

Specific examples of the aldehydes (6) include the following compounds:

-   -   saturated or unsaturated aliphatic aldehydes (6-1),    -   carbocyclic aldehydes (6-2),    -   heterocyclic aldehydes (6-3), and    -   mixtures of two or more aldehydes selected from the above three        aldehydes (6-4).

Specific examples of the saturated or unsaturated aliphatic aldehydes(6-1) include, for example, saturated or unsaturated aliphatic aldehydesof 1 to 40 carbons (the saturated or unsaturated aliphatic moiety may beeither straight or branched, and the unsaturated bond may be at anyposition) such as formaldehyde, acetaldehyde, propionaldehyde,butylaldehyde, isobutylaldehyde, valeraldehyde, isovaleraldehyde,pivalinaldehyde, capronaldehyde, heptoaldehyde, caprylaldehyde,pelargonaldehyde, caprinaldehyde, undecylaldehyde, laurinaldehyde,tridecyl aldehyde, myristinaldehyde, pentadecylaldehyde,palmitinaldehyde, margarinaldehyde, stearinaldehyde, acrolein,crotonaldehyde, propyolaldehyde, glyoxal and succindialdehyde, and thelike.

Specific examples of the carbocyclic aldehydes (6-2) include, forexample, carbocyclic aldehydes of 1 to 40 carbons such as benzaldehyde,o-tolualdehyde, m-tolualdehyde, p-tolualdehyde, salicylaldehyde,cinnamaldehyde, α-naphthaldehyde and β-naphthaldehyde, and the like.They may have a saturated or unsaturated aliphatic group (the saturatedor unsaturated aliphatic moiety may be either straight or branched, theunsaturated bond may be at any position, and the position and number ofthe substituent are not limited).

Further, specific examples of the heterocyclic aldehydes (6-3) include,for example, heterocyclic aldehydes of 1 to 40 carbons such as furfural,and the like. They may have a saturated or unsaturated aliphatic group(the saturated or unsaturated aliphatic moiety may be either straight orbranched, the unsaturated bond may be at any position, and the positionand number of the substituent are not limited).

The carbonates (7) are oxygen-containing organic compounds of thefollowing formula (VII).R—(O—COO—R′)_(n)  (VII)

For example, the carbonates (7) are compounds having one or morecarbonate bond.

Specific examples of the carbonates (7) include, for example, carbonatesof 1 to 40 carbons having a saturated or unsaturated aliphatic group, acarbocyclic ring, a carbocyclic ring with a saturated or unsaturatedaliphatic group, a saturated or unsaturated aliphatic group with acarbocyclic ring or the like (the saturated or unsaturated aliphaticmoiety may be either straight or branched, the unsaturated bond may beat any position, and the position and number of the substituent are notlimited), such as dimethyl carbonate, diethyl carbonate, di-n-propylcarbonate, diisopropyl carbonate, di-n-butyl carbonate, diisobutylcarbonate, di-tert-butyl carbonate, dipentyl carbonate, dihexylcarbonate, diheptyl carbonate, dioctyl carbonate, dinonyl carbonate,didecyl carbonate, diundecyl carbonate, didodecyl carbonate, ditridecylcarbonate, ditetradecyl carbonate, dipentadecyl carbonate, dihexadecylcarbonate, diheptadecyl carbonate, dioctadecyl carbonate and diphenylcarbonate; hydroxy(poly)oxyalkylene carbonates obtained by adding analkylene oxide to these carbonates; and the like.

Examples of derivatives of the oxygen-containing organic compounds (1)to (7) include, for example, compounds obtained by reacting theabove-described oxygen-containing organic compounds with anitrogen-containing compound, a phosphor-containing compound, elementsulfur or a sulfur-containing compound, a boron-containing compound,element halogen or a halogen-containing compound, or metal or ametal-containing compound (regardless of an organic or inorganiccompound), but are not limited thereto. The above-described compoundsused for obtaining a derivative are generally used as an additive, butthe advantageous effects thereof are not limited even when they are usedas base oil.

In the formulae (I) to (VII), R and R′ are each independently ahydrocarbon group such as alkyl, alkenyl, alkylene, cycloalkyl,alkylcycloalkyl, aryl, alkylaryl and arylalkyl (these hydrocarbon groupsmay further have one or more group or bond selected from hydroxyl group,carboxyl group, carbonyl group, ester bond and ether bond, and may alsohas an element other than carbon, hydrogen and oxygen, such as nitrogen,sulfur (e.g. a heterocyclic compound), halogen (fluorine, chlorine,etc.), phosphor, boron and metal). The carbon number of thesehydrocarbon groups is not particularly limited, but is preferably from 1to 40, more preferably from 2 to 30, particularly from 3 to 20.

Examples of the alkyl group include alkyl groups of 1 to 40 carbons suchas methyl, ethyl, n-propyl, isopropyl, n-butyl, isobutyl, sec-butyl,tert-butyl, straight or branched pentyl, straight or branched hexyl,straight or branched heptyl, straight or branched octyl, straight orbranched nonyl, straight or branched decyl, straight or branchedundecyl, straight or branched dodecyl, straight or branched tridecyl,straight or branched tetradecyl, straight or branched pentadecyl,straight or branched hexadecyl, straight or branched heptadecyl,straight or branched octadecyl, straight or branched nonadecyl, straightor branched icosyl, straight or branched henicosyl, straight or brancheddocosyl, straight or branched tricosyl and straight or branchedtetracosyl. Preferred are alkyl groups of 2 to 30 carbons, particularlyalkyl groups of 3 to 20 carbons.

Examples of the alkenyl group include alkenyl groups of 2 to 40 carbonssuch as vinyl group, straight or branched propenyl, straight or branchedbutenyl, straight or branched pentenyl, straight or branched hexenyl,straight or branched heptenyl, straight or branched octenyl, straight orbranched nonenyl, straight or branched decenyl, straight or branchedundecenyl, straight or branched dodecenyl, straight or branchedtridecenyl, straight or branched tetradecenyl, straight or branchedpentadecenyl, straight or branched hexadecenyl, straight or branchedheptadecenyl, straight or branched octadecenyl, straight or branchednonadecenyl, straight or branched icosenyl, straight or branchedheneicosenyl, straight or branched docosenyl, straight or branchedtricosenyl and straight or branched tetracosenyl. Preferred are alkenylgroups of 2 to 30 carbons, particularly alkenyl groups of 3 to 20carbons.

Examples of the cycloalkyl group include cycloalkyl groups of 3 to 40carbons such as cyclopentyl, cyclohexyl, cycloheptyl and cyclooctyl.Preferred are cycloalkyl groups of 3 to 20 carbons, particularlycycloalkyl groups of 5 to 8 carbons.

Examples of the alkylcycloalkyl group include alkylcycloalkyl groups of4 to 40 carbons such as methylcyclopentyl, dimethylcyclopentyl(including all structural isomers), methylethylcyclopentyl (includingall structural isomers), diethylcyclopentyl (including all structuralisomers), methylcyclohexyl, dimethylcyclohexyl (including all structuralisomers), methylethylcyclohexyl (including all structural isomers),diethylcyclohexyl (including all structural isomers), methylcycloheptyl,dimethylcycloheptyl (including all structural isomers),methylethylcycloheptyl (including all structural isomers) anddiethylcycloheptyl (including all structural isomers). Preferred arealkylcycloalkyl groups of 5 to 20 carbons, particularly alkylcycloalkylgroups of 6 to 12 carbons.

Examples of the aryl group include a phenyl group, a naphthyl group,aryl groups of 6 to 20 carbons. Preferred are aryl groups of 6 to 10carbons.

Examples of the alkylaryl group include monosubstituted phenyl groupssuch as a tolyl (including all structural isomers), ethylphenyl(including all structural isomers), straight or branched propylphenyl(including all structural isomers), straight or branched butylphenyl(including all structural isomers), straight or branched pentylphenyl(including all structural isomers), straight or branched hexylphenyl(including all structural isomers), straight or branched heptylphenyl(including all structural isomers), straight or branched octylphenyl(including all structural isomers), straight or branched nonylphenyl(including all structural isomers), straight or branched decylphenyl(including all structural isomers), straight or branched undecylphenyl(including all structural isomers) and straight or brancheddodecylphenyl (including all structural isomers); aryl groups having thesame or different two or more straight or branched alkyl groups (thealkyl groups may further have an aryl group, an alkylaryl group or anarylalkyl group and includes all structural isomers) such as xylyl(including all structural isomers), diethylphenyl, dipropylphenyl,2-methyl-6-tert-butylphenyl, 2,6-di-tert-butyl-4-methylphenyl and2,6-di-tert-butyl-4-(3,5-di-tert-butyl-4-benzyl)phenyl; and the like.The alkylaryl group is an alkylaryl group of 7 to 40 carbons, preferablyan alkylaryl group of 7 to 20 carbons, particularly an alkylaryl groupof 7 to 12 carbons.

Examples of the arylalkyl group include arylalkyl groups of 7 to 40carbons such as benzyl, phenylethyl, phenylpropyl (including isomers ofthe propyl moiety), phenylbutyl (including isomers of the butyl moiety),phenylpentyl (including isomers of the pentyl moiety) and phenylhexyl(including isomers of the hexyl moiety). Preferred are arylalkyl groupsof 7 to 20 carbons, particularly arylalkyl groups of 7 to 12 carbons.

Derivatives of the above-described oxygen-containing organic compoundscan also be equally used. Specific examples of such derivatives include,for example, sulfides of a compound selected from the above-describedalcohols, carboxylic acids, esters, ethers, ketones, aldehydes andcarbonates; halides (fluorides, chlorides, etc.) of such a compound;reaction products of such a compound with sulfuric acid, nitric acid,boric acid, phosphoric acid or an ester or metal salt of such acids;reaction products of such a compound with metal, a metal-containingcompound or a amine compound; and the like. Among them, preferredexamples are reaction products of one or more compound selected from theabove-described alcohols, carboxylic acids, aldehydes and thederivatives thereof with an amine compound (e.g. Mannich reactionproducts, acylation products, amides, etc.).

Examples of such amine compounds include ammonia, monoamines, diaminesand polyamines. More specific examples are ammonia; alkylamines havingan alkyl group of 1 to 30 carbons (the alkyl group may be eitherstraight or branched) such as methylamine, ethylamine, propylamine,butylamine, pentylamine, hexylamine, heptylamine, octylamine,nonylamine, decylamine, undecylamine, dodecylamine, tridecylamine,tetradecylamine, pentadecylamine, hexadecylamine, heptadecylamine,octadecylamine, stearylamine, dimethylamine, diethylamine,dipropylamine, dibutylamine, dipentylamine, dihexylamine, diheptylamine,dioctylamine, dinonylamine, didecylamine, diundecylamine,didodecylamine, ditridecylamine, ditetradecylamine, dipentadecylamine,dihexadecylamine, diheptadecylarnine, dioctadecylamine,methylethylamine, methylpropylamine, methylbutylamine, ethylpropylamine,ethylbutylamine and propylbutylamine; alkenylamines having an alkenylgroup of 2 to 30 carbons (the alkenyl group may be either straight orbranched) such as ethenylamine, propenylamine, butenylamine,octenylamine and oleylamine; alkanolamines having an alkanol group of 1to 30 carbons (the alkanol group may be either straight or branched)such as methanolamine, ethanolamine, propanolamine, butanolamine,pentanolamine, hexanolamine, heptanolamine, octanolamine, nananolamine,methanolethanolamine, methanolpropanolamine, methanolbutanolamine,ethanolpropanolamine, ethanolbutanolamine and propanolbutanolamine;alkylenediamines having an alkylene group of 1 to 30 carbons such asmethylenediamine, ethylenediamine, propylenediamine and butylenediamine;polyamines such as diethylenetriamine, triethylenetetramine,tetraethylenepentamine and pentaethylenehexamine; the above-describedmonoamines, diamines and polyamines further having an alkyl or alkenylgroup of 8 to 20 carbons, such as undecyldiethylamine,undecyldiethanolamine, dodecyldipropanolamine, oleyldiethanolamine,oleylpropylenediamine and stearyltetraethylenepentamine; heterocycliccompounds such as N-hydroxyethyloleylimidazoline; alkyleneoxide adductsof these compounds; mixtures of these compounds; and the like.

Among these nitrogen compounds, preferred examples include aliphaticamines having an alkyl or alkenyl group of 10 to 20 carbons (they may beeither straight or branched) such as decylamine, dodecylamine,tridecylamine, heptadecylamine, octadecylarnine, oleylamine andstearylamine. Further, among these derivatives of the oxygen-containingorganic compounds, preferred examples include carboxylic amides of 8 to20 carbons such as oleic amide.

The above-described oxygen-containing organic compounds exhibit verygood low-friction property even when they are used alone (i.e. 100 mass%) in a sliding surface between the hard carbon layer of the slidingmember and a counterpart siding member of an arbitrary material.Alternatively, a lubiricant composed of such an oxygen-containingorganic compound and a lubricant base (e.g. base oil) or an additiveadded thereto, or a lubiricant composed of a medium (including alubricant base with or without an additive) and such anoxygen-containing organic compound added thereto may be applied to thesliding surface for lubrication.

Specifically, such lubricants are produced by adding anoxygen-containing organic compound to a medium such as mineral oil,synthetic oil, natural fat, diluent oil, grease, wax, a hydrocarbon of 3to 40 carbons, hydrocarbon solvent, organic solvent other thanhydrocarbons, water and mixtures thereof, particularly a medium that isin the form of liquid, grease or wax in the sliding condition or atordinary temperature. Regarding the content of the oxygen-containingorganic compound included in the medium, the lower limit thereof is notparticularly limited but is typically 0.001 mass %, preferably 0.05 mass%, more preferably 0.1 mass %, and the content may be more than 3.0 mass%. Further, the upper limit thereof is 100 mass % as described above.However, it is preferably 50 mass %, more preferably 20 mass %, yet morepreferably 10 mass %, particularly 5 mass %. Even when theoxygen-containing organic compound is included in a low amount like from0.1 to 2 mass %, it can exhibit a good low-friction property.

FIG. 3 are (a) a plan view and (b) a partial cross-sectional view takenin the radial direction of a piston ring 1′, which is an example of thesliding member 1 in which the predetermined hard chromium plated layer 4and the predetermined hard carbon layer 6 are formed on the outerperipheral surface (side surface) of the base 2. In this example, thehard chromium plated layer 4 and the hard carbon layer 6 are laminatedon the sliding surface of the base 2 in the written order. With thisconfiguration, when the outer peripheral surface (side surface) of thepiston ring slides on a counterpart cylinder bore (not shown) and afriction is caused, it can exhibit good peeling resistance and goodlow-friction property.

FIG. 4 are (a) a plan view and (b) a partial cross sectional view takenin the radial direction of a piston ring 1″, which is an example of thesliding member 1 in which the predetermined hard chromium plated layer 4and the predetermined hard carbon layer 6 are formed on the outerperipheral surface (side surface) and the upper and lower surfaces ofthe base 2. In this example, the hard chromium plated layer 4 and thehard carbon layer 6 are laminated on the sliding surface and the contactsurfaces of the base 2 in the written order. With this configuration,when the outer peripheral surface (side surface) of the piston ringslides on a counterpart cylinder bore (not shown) and a friction iscaused, it can exhibit peeling resistance and low-friction property.Further, when the upper or lower surface of the piston ring come incontact with a groove of a piston, which is a counterpart member (notshown), it can exhibit low-friction property and seizing resistance.Although not shown in the figure, it is also possible that the hardchromium plated layer and the hard carbon layer are laminated on theoverall surface of the piston ring in the written order. Further, it isalso suitable that the hard chromium plated layer and the hard carbonlayer are laminated in the written order on the sliding surface of thecylinder bore, which slides against the piston ring.

Next, particularly preferred lubricants used for the sliding member ofthe present invention will be described in detail.

The lubricant, which is a lubricating oil composition used incombination with the piston ring for an automobile engine, containslubricant base oil and a component added thereto selected from fattyacid ester type ash-free friction adjusters, aliphatic amine typeash-free friction adjusters, polybutenylsuccinimide,polybutenylsuccinimide derivatives, zinc dithiophosphate and anycombination thereof.

The lubricant base oil is not particularly limited, and may be of anytype that is generally used as base oil of a lubricating oilcomposition, such as mineral oil, synthetic oil, fat and a mixturethereof. The lubricant base oil may also be composed of a combination oftwo or more mineral base oils or two or more synthetic base oils. Themixing ratio of two or more base oils of the mixture is not particularlylimited and may be arbitrarily selected.

Specific examples of mineral oils that can be used include paraffin oilsand naphthene oils obtained by distilling crude oil at atmosphericpressure and a reduced pressure and then further purifying the obtainedlubricating oil distillate by a suitable combination of purificationtreatments such as solvent deasphalting, solvent extraction,hydrocracking, solvent dewaxing, hydropurification, sulfuric acidcleaning and gray clay treatment, as well as normal paraffins, and thelike. Among them, typical examples are mineral oils produced by solventpurification and hydropurification. However, it is more preferred to useisoparaffins that are produced by a high hydrocracking process or GTLwax (gas-to-liquid wax) isomerization, which can further reduce thearomatic component.

Specific examples of the synthetic oil include poly-α-olefins (e.g.1-octen oligomer, 1-decene oligomer, ethylene-propylene oligomer, etc.),hydrogenated poly-α-olefins, isobutene oligomer, hydrogenated isobuteneoligomer, isoparaffin, alkylbenzenes, alkylnaphthalenes, diesters (e.g.ditridecyl glutarate, dioctyl adipate, diisodecyl adipate, ditridecyladipate, dioctyl sebacate, etc.) polyol esters (e.g. trimethylolpropaneesters such as trimethylolpropane caprylate, trimethylolpropanepelargonate and trimethylolpropane isostearate; and pentaerythritolesters such as pentaerythritol 2-ethylhexanoate and pentaerythritolpelargonate), polyoxyalkylene glycols, dialkyldiphenyl ethers,polyphenyl ether, and the like. Among them, preferred examples arepoly-α-olefins such as 1-octene oligomer and 1-decene oligomer, and thehydrogenated products thereof. Further, among these esters, polyolesters are particularly preferred. Typically, such synthetic oilsinclude gear oils, automobile engine oils, transmission oils, turbineoils and spindle oils, and the like.

The sulfur content in the lubricant base oil is not particularlylimited, but is preferably equal to or less than 0.2 mass %, morepreferably equal to or less than 0.1 mass %, yet more preferably equalto or less than 0.05 mass % with respect to the total amount of thelubricant base oil. Since hydropurified mineral oil and synthetic baseoil contain 0.005 mass % or less of sulfur or are substantiallysulfur-free (5 ppm or less), they are preferably used as the base oil.

The aromatic content in the lubricant base oil is not particularlylimited. However, in order to maintain the low-friction property of thelubricating oil composition for a long time, it is preferably equal toor less than 15 mass %, more preferably equal to or less than 10 mass %,yet more preferably equal to or less than 5 mass %. That is, it is notpreferred that the total aromatic content in the lubricant base oil isgreater than 15 mass %, since it results in poor oxidation stability. Asused herein, the term “total aromatic content” means aromatic fractioncontent measured according to ASTM D2549. The aromatic fractiontypically contains alkylbenzenes, alkylnaphthalenes, anthracene,phenanthrene, alkylates of these compounds, condensates of four or morebenzene rings, heteroaromatic compounds such as pyridines, quinolines,phenols and naphthols, and the like.

The kinetic viscosity of the lubricant base oil is not particularlylimited, either. However, when it is used for the lubricating oilcomposition, the kinetic viscosity at 100° C. is preferably equal to orgreater than 2 mm²/s, more preferably equal to or greater than 3 mm²/s.Further, the kinetic viscosity at 100° C. is preferably equal to or lessthan 20 mm²/s, more preferably equal to or less than 10 mm²/s,particularly equal to or less than 8 mm²/s. It is not preferred that thekinetic viscosity at 100° C. is less than 2 mm²/s, since it may resultin insufficient wear resistance and poor evaporation property. It is notpreferred that the kinetic viscosity is greater than 20 mm²/s, since itmay result in a difficulty in exhibiting low-friction property and poorlow-temperature performance Any mixture of two or more base oil selectedfrom the above-described base oil may also be used. As long as thekinetic viscosity at 100° C. is within the above-described preferredrange, individual base oils may solely have a kinetic viscosity beyondthe above-described range. When the lubricant base oil has a kineticviscosity at 100° C. of 2 mm²/s or more, it becomes possible to obtainthe composition that can form adequate oil film, has good lubricity andalso has low evaporation loss of base oil in a high-temperaturecondition. When the lubricant base oil has a kinetic viscosity at 100°C. of 20 mm²/s or less, it becomes possible to obtain the compositionthat exhibits further low friction at a lubrication point due to itsreduced flow resistance.

The viscosity index of the lubricant base oil is not particularlylimited, but is preferably equal to or more than 80. When it is used forthe lubricating oil composition, the viscosity index is preferably equalto or greater than 100, more preferably equal to or greater than 120 ormay be from 140 to 250. When the lubricant base oil with high viscosityindex is used, it becomes possible to obtain the composition that hasgood friction reducing effect as well as low oil consumption and goodlow-temperature viscosity profile.

Examples of fatty acid ester type ash-free friction adjusters and/oraliphatic amine type ash-free friction adjusters that can be usedinclude fatty acid esters and aliphatic amine compounds having astraight or branched hydrocarbon group of 6 to 30 carbons, preferably 8to 24 carbons, particularly 10 to 20 carbons, and any combinationthereof. When the number of carbon is out of the range of 6 to 30, itmay result in insufficient friction reducing effect.

Specific examples of such straight or branched hydrocarbon groups of 6to 30 carbons include alkyl groups such as hexyl, heptyl, octyl, nonyl,decyl, undecyl, dodecyl, tridecyl, tetradecyl, pentadecyl, hexadecyl,heptadecyl, octadecyl, nonadecyl, icosyl, henicosyl, docosyl, tricosyl,tetracosyl, pentacosyl, hexacosyl, heptacosyl, octacosyl, nonacosyl andtriacontyl; alkenyl groups such as hexenyl, heptenyl, octenyl, nonenyl,decenyl, undecenyl, dodecenyl, tridecenyl, tetradecenyl, pentadecenyl,hexadecenyl, heptadecenyl, octadecenyl, nonadecenyl, icosenyl,heneicosenyl, docosenyl, tricosenyl, tetracosenyl, pentacosenyl,hexacosenyl, heptacosenyl, octacosenyl and nocosenyl and triacontenyl;and the like. The above-described alkyl and alkenyl groups include allpossible respective straight and branched structures, and the doublebond of the alkenyl groups may be at any position.

Examples of the fatty acid esters include esters of a fatty acid havingthe above-described hydrocarbon group of 6 to 30 carbons and a mono- orpolyhydric aliphatic alcohol. Specifically, particularly preferredexamples include glycerin monooleate, glycerin dioleate, sorbitanmonooleate, sorbitan dioleate and the like.

Examples of the aliphatic amine compounds include aliphatic monoaminesand the alkylene oxide adducts thereof, aliphatic polyamines,imidazoline compounds and the derivatives thereof, and the like.Specific examples include aliphatic amine compounds such as laurylamine,lauryldiethylamine, lauryldiethanolamine, dodecyldipropanolamine,palmitylamine, stearylamine, stearyltetraethylenepentamine, oleylamine,oleylpropylenediamine, oleyldiethanolamine andN-hydroxyethyloleylimidazoline; aminealkyleneoxide adducts (e.g.N,N-dipolyoxyalkylene-N-alkyl (or alkenyl) (6 to 28 carbons)) of thesealiphatic amine compounds; so-called acid-modified compounds of thesealiphatic amine compounds, in which the residual amino and/or iminogroup is fully or partly neutralized or amidated by a reaction with amonocarboxylic acid (fatty acid etc.) of 2 to 30 carbons or apolycarboxylic acid of 2 to 30 carbons such as oxalic acid, phthalicacid, trimellitic acid and pyromellitic acid. Suitable examples areN,N-dipolyoxyethylene-N-oleylamine and the like.

The content of the fatty acid ester type ash-free friction adjusterand/or the aliphatic amine type ash-free friction adjuster in thelubricating oil composition is not particularly limited, but ispreferably from 0.05 mass % to 3.0 mass %, more preferably from 0.1 mass% to 2.0 mass %, particularly from 0.5 mass % to 1.4 mass % with respectto the total amount of the composition. The content less than 0.05 mass% is unfavorable since the friction reducing effect is likely to bedecreased. Further, the content greater than 3.0 mass % is unfavorablesince the solubility to lubricating oil and the storage stability areseriously deteriorated, which are likely to cause a precipitate.

Examples of the above-described polybutenylsuccinimides include thecompounds of the following formulae (VIII) and (IX).

In the formulae, PIB is a polybutenyl group, which is derived from apolybutene having a number average molecular weight of from 900 to 3500,desirably from 1000 to 2000 obtained by polymerizing high-purityisobutene or a mixture of 1-butene and isobutene using a fluoboriccatalyst or an aluminum chloride catalyst. The number average molecularweight of less than 900 is unfavorable since the cleaning effect islikely to be deteriorated. Further, the number average molecular weightof greater than 3500 is unfavorable since the low-temperature fluidityis likely to be deteriorated. Further, in the formulae, n is an integerof 1 to 5, more preferably 2 to 4 in terms of achieving good cleaningproperty. It is also preferable that the polybutene is purifiedbeforehand by adsorption or sufficient washing with water so that thetrace amount of residual fluorine or chlorine, which is derived from thecatalyst used in the production process, is removed to a concentrationof 50 mass ppm or less, preferably 10 mass ppm or less, particularly 1mass ppm or less.

The production method of the polybutenylsuccinimides is not particularlylimited. For example, they may be produced by reacting a chloride of theabove-described polybutene or the purified polybutene, from whichchlorine and fluorine is sufficiently removed, with maleic acidanhydride at a temperature of 100° C. to 200° C. yielding a polybutenylsuccinic acid and then reacting the polybutenyl succinic acid with apolyamine such as diethylenetriamine, triethylenetetramine,tetraethylenepentamine and pentaethylenehexamine.

Examples of the polybutenylsuccinimide derivatives include so-calledboron-modified or acid-modified compounds obtained by reacting acompound of the above formula (VIII) or (XI) with a boron compound or anoxygen-containing organic compound so that the residual amino and/orimino group is partly or fully neutralized or amidated. Among them,boron-containing polybutenylsuccinimide, particularly boron-containingbispolybuthenylsuccinimide is currently the most preferred.

Examples of the boron compound include boric acids, borates, and boricacid esters. Specific examples of such boric acids include orthoboricacid, metaboric acid, tetraboric acid and the like. Examples of suchborates include ammonium salts and the like. Specifically, preferredexamples include ammonium borates such as ammonium metaborate, ammoniumtetraborate, ammonium pentaborate and ammonium octaborate. Examples ofsuch boric acid esters include esters of boric acid and an alkyl alcoholof preferably 1 to 6 carbons. To be more specific, preferred examplesinclude monomethyl borate, dimethyl borate, trimethyl borate, monoethylborate, diethyl borate, triethyl borate, monopropyl borate, dipropylborate, tripropyl borate, monobutyl borate, dibutyl borate, tributylborate and the like. The mass ratio “B/N” of boron content B to nitrogencontent N in the boron-containing polybutenylsuccinimide is typicallyfrom 0.1 to 3, preferably from 0.2 to 1.

Specific examples of the oxygen-containing organic compound includemonocarboxylic acids of 1 to 30 carbons such as formic acid, aceticacid, glycolic acid, propionic acid, lactic acid, butyric acid, valeicacid, caproic acid, enanthic acid, caprylic acid, pelargonic acid,capric acid, undecylic acid, lauric acid, tridecanic acid, myristicacid, pentadecanic acid, palmitic acid, margaric acid, stearic acid,oleic acid, nonadecanic acid and eicosanic acid; polycarboxylic acids of2 to 30 carbons such as oxalic acid, phthalic acid, trimellitic acid andpyromellitic acid; anhydrides and ester compounds of these compounds,alkylene oxides of 2 to 6 carbons, hydroxy(poly)oxyalkylenecarbonates,and the like.

In the lubricating oil composition, the content of thepolybutenylsuccinimide and/or the polybutenylsuccinimide derivative isnot particularly limited, but is preferably from 0.1 mass % to 15 mass%, more preferably from 1.0 mass % to 12 mass %. When the content isless than 0.1 mass %, the cleaning effect may be poor. When the contentis greater than 15 mass %, it is difficult to further increase thecleaning effect, and the demulsification effect is likely to bedeteriorated.

Further, it is preferred that the lubricating oil composition contains azinc dithiophosphate of the following formula (X).

In the formula (X), R⁴, R⁵, R⁶ and R⁷ are each independently ahydrocarbon group of 1 to 24 carbons. Such hydrocarbon groups includestraight or branched alkyl groups of 1 to 24 carbons, straight orbranched alkenyl groups of 3 to 24 carbons, cycloalkyl or straight orbranched alkylcycloalkyl groups of 5 to 13 carbons, aryl or straight orbranched alkylaryl groups of 6 to 18 carbons, arylalkyl groups of 7 to19 carbons and the like. The alkyl or alkenyl groups may be any ofprimary, secondary and tertiary groups.

Specific examples of R⁴, R⁵, R⁶ and R⁷ include alkyl groups such asmethyl, ethyl, propyl, butyl, pentyl, hexyl, heptyl, octyl, nonyl,decyl, undecyl, dodecyl, tridecyl, tetradecyl, pentadecyl, hexadecyl,heptadecyl, octadecyl, nonadecyl, icosyl, henicosyl, docosyl, tricosyland tetracosyl; alkenyl groups such as propenyl, isopropenyl, butenyl,butadienyl, pentenyl, hexenyl, heptenyl, octenyl, nonenyl, decenyl,undecenyl, dodecenyl, tridecenyl, tetradecenyl, pentadecenyl,hexadecenyl, heptadecenyl, octadecenyl (e.g. oleyl, etc.), nonadecenyl,icosenyl, heneicosenyl, docosenyl, tricosenyl and tetracosenyl;cycloalkyl groups such as cyclopentyl, cyclohexyl and cycloheptyl;alkylcycloalkyl groups such as methylcyclopentyl, dimethylcyclopentyl,ethylcyclopentyl, propylcyclopentyl, ethylmethylcyclopentyl,trimethylcyclopentyl, diethylcyclopentyl, ethyldimethylcyclopentyl,propylmethylcyclopentyl, propylethylcyclopentyl, di-propylcyclopentyl,propylethylmethylcyclopentyl, methylcyclohexyl, dimethylcyclohexyl,ethylcyclohexyl, propylcyclohexyl, ethylmethylcyclohexyl,trimethylcyclohexyl, diethylcyclohexyl, ethyldimethylcyclohexyl,propylmethylcyclohexyl, propylethylcyclohexyl, di-propylcyclohexyl,propylethylmethylcyclohexyl, methylcycloheptyl, dimethylcycloheptyl,ethylcycloheptyl, propylcycloheptyl, ethylmethylcycloheptyl,trimethylcycloheptyl, diethylcycloheptyl, ethyldimethylcycloheptyl,propylmethylcycloheptyl, propylethylcycloheptyl, di-propylcycloheptyland propylethylmethylcycloheptyl; aryl groups such as phenyl andnaphthyl; alkylaryl groups such as tolyl, xylyl, ethylphenyl,propylphenyl, ethylmethylphenyl, trimethylphenyl, butylphenyl,propylmethylphenyl, diethylphenyl, ethyldimethylphenyl,tetramethylphenyl, pentylphenyl, hexylphenyl, heptylphenyl, octylphenyl,nonylphenyl, decylphenyl, undecylphenyl and dodecylphenyl; arylalkylgroups such as benzyl, methylbenzyl, dimetnylbenzyl, phenethyl,methylphenethyl and dimethylphenethyl; and the like. These hydrocarbongroups that can be R⁴, R⁵, R⁶ or R⁷ include all possible straight andbranched structures. Further, the double bond of the alkenyl groups maybe at any position. The alkyl group on the cycloalkyl groups, the alkylgroup on the aryl groups, and the aryl group on the alkyl groups may bebound at any position. Among these hydrocarbon groups, straight orbranched alkyl groups of 1 to 18 carbons, and aryl or straight orbranched alkylaryl groups of 6 to 18 carbons are particularly preferred.

Preferred specific examples of the zinc dithiophosphate include zincdiisopropyl dithiophosphate, zinc diisobutyl dithiophosphate, zincdi-sec-butyl dithiophosphate, zinc di-sec-pentyl dithiophosphate, zincdi-n-hexyl dithiophosphate, zinc di-sec-hexyl dithiophosphate, zincdioctyl dithiophosphate, zinc di-2-ethylhexyl dithiophosphate, zincdi-n-decyl dithiophosphate, zinc di-n-dodecyl dithiophosphate, zincdiisotridecyl dithiophosphate, and mixtures composed of any combinationthereof, and the like.

The content of the zinc dithiophosphate is not particularly limited.However, in terms of achieving higher friction reducing effect, thecontent is preferably equal to or less than 0.1 mass %, more preferablyequal to or less than 0.06 mass % in phosphor-element equivalent withrespect to the total amount of the composition. It is particularlypreferred that no zinc dithiophosphate is contained. When the content ofthe zinc dithiophosphate is greater than 0.1 mass % in phosphor-elementequivalent with respect to the total amount of the composition, it mayinhibit the friction reducing effect of the above-described fatty acidester type ash-free friction adjuster and aliphatic amine type ash-freefriction adjuster in a sliding surface between the member with the hardcarbon film coating and a counterpart sliding member of an arbitrarymaterial.

The zinc dithiophosphate may be produced by any conventional method, andthe production method is not particularly limited. Specifically, it canbe synthesized by reacting an alcohol or phenol having hydrocarbongroups corresponding to R⁴, R⁵, R⁶ and R⁷ with diphosphorus pentoxide(P₂O₅) to yield a dithiophosphoric acid, and then neutralizing it withzinc oxide. It should be understood that the structure of the zincdithiophosphate differs depending on the alcohol material used. Amixture of two or more zinc dithiophosphates of the formula (X) in anarbitrary mixing ratio may also be used.

As described above, the lubricating oil composition exhibits remarkablyhigh low-friction property when it is used in a sliding surface betweenthe sliding member with the hard carbon layer coating and a counterpartsliding member of an arbitrary material. In order to enhance theperformance required for internal combustion engine lubricating oilcompositions, a metal cleaner, an antioxidant, a viscosity indexenhancer, another ash-free friction adjuster, another ash-freedispersant, an anti-wear agent or extreme pressure agent, a rustinhibitor, a non-ionic surfactant, a demulsifier, a metal deactivator,an antifoam agent or the like may be added alone or in combination.

Metal cleaners that can be used include any compound that is generallyused as a metal cleaner for lubricating oil. For example, sulfonates,phenates, salicylates, naphthenates and the like of alkaline metals oralkaline earth metals may be used alone or in combination. Examples ofsuch alkali metals include sodium (Na), potassium (K) and the like, andexamples of such alkali earth metals include calcium (Ca), magnesium(Mg) and the like. Further, preferred specific examples includesulfonates, phenates, and salicylates of Ca or Mg. The total base valueof the metal cleaner and the amount thereof added are suitably selectedaccording to the required performance of the lubricating oil. The totalbase value is typically from 0 to 500 mg KOH/g, preferably from 150 to400 mg KOH/g measured by the perchloric acid method, and the amountadded is typically from 0.1 mass % to 10 mass % with respect to thetotal amount of the composition.

Antioxidants that can be used include any compound that is generallyused as an antioxidant for lubricating oil. For example, examples ofsuch antioxidants include phenol antioxidants such as4,4′-methylenebis(2,6-di-tert-butylphenol),octadecyl-3-(3,5-di-tert-butyl-4-hydroxyphenyl)propionate andoctadecyl-3-(3-methyl-5-tert-butyl-4-hydroxyphenyl)propionate; amineantioxidants such as phenyl-α-naphthylamine, alkylphenyl-α-naphtylaminesand alkyldiphenylamines; mixtures composed of any combination ofthereof; and the like. The amount of the antioxidant added is typicallyfrom 0.01 mass % to 5 mass % with respect to the total amount of thecomposition.

Specific examples of the viscosity index enhancer include so-callednon-dispersed viscosity index enhancers such as copolymers of one ormore monomer selected from various methacrylates and the hydrogenatedproducts thereof, so-called dispersed viscosity index enhancers that areobtained by further copolymerizing a monomer selected from variousmethacrylates having a nitrogen compound, and the like. Further,specific examples of the viscosity index enhancer also includenon-dispersed or dispersed ethylene-α-olefin copolymers (where theα-olefin is propylene, 1-butene, 1-pentene and the like) and thehydrogenated products thereof, polyisobutylene and the hydrogenatedproduct thereof, styrene-diene hydrogenated copolymers, styrene-maleicanhydride ester copolymers, polyalkylstyrene and the like. It isrequired to select the molecular weight of these viscosity indexenhancers in view of the shear stability. Specifically, among theseviscosity index enhancers, the average number molecular weight ofdispersed or non-dispersed polymethacrylates is from 5000 to 1000000,preferably from 100000 to 800000, the number average molecular weight ofpolyisobutylene and the hydrogenated product thereof is from 800 to5000, the number average molecular weight of ethylene-α-olefincopolymers and the hydrogenated products thereof is from 800 to 300000,preferably from 10000 to 200000. Such viscosity index enhancers may beadded alone or in combination of plural types, and the content istypically from 0.1 mass % to 40.0 mass % with respect to the lubricatingoil composition.

Examples of another ash-free friction adjuster include ash-free frictionadjusters such as borates, higher alcohols and fatty acid ethers; metalfriction adjusters such as molybdenum dithiophosphate, molybdenumdithiocarbamate and molybdenum disulfide; and the like.

Examples of another ash-free dispersant include polybutenylbenzylamineand polybutenylamine having a polybutenyl group with a number averagemolecular weight of from 900 to 3500, polybutenylsuccinimide having apolybutenyl group with a number average molecular weight less than 900,the derivatives thereof and the like.

Examples of the anti-wear agent and the extreme pressure agent includedisulfides, sulfurized greases, sulfurized olefins, phosphates havingone to three hydrocarbon group of 2 to 20 carbons, thiophosphates,phosphites, thiophosphites, the amine salts thereof, and the like.

Examples of the rust inhibitor include alkylbenzene sulfonates,dinonylnaphthalene sulfonate, alkyenylsuccinates, polyol esters and thelike.

Examples of the non-ionic surfactant or the demulsifier includepolyalkylene glycol non-ionic surfactants such as polyoxyethylenealkylethers, polyoxyethylene alkylphenylethers and polyoxyethylenealkylnaphthylethers. Further, examples of the metal deactivator includeimidazoline, pyrimidine derivatives, thiadiazoles, benzotriazoles,thiadiazoles and the like.

Examples of the antifoam agent include silicone, fluorosilicone,fluoroalkylethers and the like.

When such additives are added to the lubricating oil composition, thecontent thereof is suitably selected within the range of 0.01 mass % to5 mass % for the another friction adjuster, another ash-free dispersant,anti-wear agents, extreme pressure agents, and demulsifiers, within therange of 0.005 mass % to 1 mass % for metal deactivators, and within therange of 0.0005 to 1 mass % for antifoam agents, with respect to thecomposition

Hereinafter, the present invention will be described in further detailwith examples and comparative examples.

Reference 1

On the surface of a piston ring base for an automobile engine, a hardchromium plated layer is formed to a thickness of approximately 100 μmby electrolytic plating. Then, the piston ring base with the hardchromium plated layer formed thereon is heated at 250° C. in the air.Thereafter, a hydrogen-free hard carbon layer is formed on the hardchromium plated layer of the piston ring base by PVD. The piston ring ofthis example was thus obtained.

After forming the hard chromium plated layer, which was a midway step ofthe process, the hydrogen concentration of the hard chromium platedlayer was measured by using another piston ring, which was prepared byforming the hard chromium plated layer in the same batch and heating itin the same condition as the production process of the above-describedpiston ring. To measure the hydrogen concentration, a sample was cut toa suitable size and placed in a crucible. The sample was heated toapproximately 1000° C., and the generated gas was analyzed. The carriergas used was argon (Ar), and the hydrogen concentration in dischargedgas was measured, and the amount of hydrogen was calculated from theflow rate of the carrier gas. At the same time, similar measurement wasperformed for a sample without the hard chromium plated layer. Bysubtracting the measured value as a background value, the amount ofhydrogen contained only in the hard chromium plated layer wascalculated. As a result, the hydrogen concentration in the hard chromiumplated layer was 148 ppm.

Example 2

A piston ring of this example was prepared in the same manner asReference 1 except that the heating temperature was 280° C. In theprocess, the hydrogen concentration of the hard chromium plated layerwas also measured by the same process.

Example 3

A piston ring of this example was prepared in the same manner asReference 1 except that the heating temperature was 300° C. In theprocess, the hydrogen concentration of the hard chromium plated layerwas also measured by the same process.

Example 4

A piston ring of this example was prepared in the same manner asReference 1 except that the heating temperature was 350° C. In theprocess, the hydrogen concentration of the hard chromium plated layerwas also measured by the same process.

(Reference 5)

A piston ring of this example was prepared in the same manner asReference 1 except that the heating temperature was 400° C. In theprocess, the hydrogen concentration of the hard chromium plated layerwas also measured by the same process.

Comparative Example 1

A piston ring of this example was prepared in the same manner asReference 1 except that the sample was not heated. In the process, thehydrogen concentration of the hard chromium plated layer was alsomeasured by the same process.

Comparative Example 2

A piston ring of this example was prepared in the same manner asReference 1 except that the heating temperature was 100° C. In theprocess, the hydrogen concentration of the hard chromium plated layerwas also measured by the same process.

Comparative Example 3

A piston ring of this example was prepared in the same manner asReference 1 except that the heating temperature was 200° C. In theprocess, the hydrogen concentration of the hard chromium plated layerwas also measured by the same process.

The configuration of the above-described examples and ComparativeExamples are shown in Table 1.

TABLE 1 Heating Hydrogen General Temperature Concentration Peeling, TestEvaluation (° C.) (ppm) Result Result Reference 1 250 148 minute peelingB Example 2 280 105 no peeling A Example 3 300 66 no peeling A Example 4350 28 no peeling A Reference 5 400 9 no peeling B Comparative notreatment 819 large peeling C Example 1 Comparative 100 762 largepeeling C Example 2 Comparative 200 319 large peeling C Example 3

(Reciprocating Sliding Test)

A reciprocating sliding test was performed for 30 minutes under theconditions of an average sliding speed of 0.3 m/s and a pressing load of100 N for each piston ring of the above-described Examples andComparative Examples. Engine oil was used as a lubricant. The surfacecondition of the hard carbon layers were observed under a scanningelectron microscope. The results are also shown in Table 1. Further,general evaluation results are also shown in Table 1. In Table 1, “A”represents practically acceptable and very good, “B” refers topractically acceptable and good, and “C” represents practicallyunacceptable and not good.

FIG. 5, FIG. 6, FIG. 7 and FIG. 8 are scanning electron micrographsillustrating the surface condition of the piston rings, which are thesliding members of Reference 1, Example 2, Comparative Example 1 andComparative Example 3 respectively.

In the piston ring of Reference 1, very minute peeling was observed inthe hard carbon layer of the sliding surface, which was howeveracceptable for practical use. It was found that the hard carbon layer isstrongly adhered, i.e. has high peeling resistance (see FIG. 5).

In the piston rings of Example 2 to Example 4, no peeling was observedin the hard carbon layers of the sliding surface. It was found that thehard carbon layers are strongly adhered, i.e. has high peelingresistance (see FIG. 6).

In the piston ring of Reference 5, no peeling was observed in the hardcarbon layer of the sliding surface. It was found that the hard carbonlayer is strongly adhered, i.e. has high peeling resistance. However,the hardness of the hard chromium plated layer was decreased by 200 HVor more compared to the untreated condition. It is presumed thatmicrocracks present in the hard chromium plating were grown, and theoriginal wear resistance of the hard chromium plated layer was therebysignificantly decreased.

In the piston rings of Comparative Example 1 to Comparative Example 3,large peeling was observed in the hard carbon layers of the slidingsurfaces. It was found that the hard carbon layers are not adequatelyadhered, and the peeling resistance is not sufficient (see FIG. 7 andFIG. 8).

While the present invention was described with a few embodiments andExamples, the present invention is not limited thereto, and variousmodifications can be made within the scope of the gist of the presentinvention.

That is, the above-described embodiments and Examples are piston rings,which are an example of the sliding member. However, the presentinvention is not limited thereto, and the present invention encompassesother sliding members such as those of a valve train or an intake andexhaust system and those of a power drive. Specific examples of suchsliding members of a valve train or an intake and exhaust system includepiston pins, pistons (or piston skirts (a piston skirt means a skirtportion of a piston), cylinders (or cylinder liners), plungers, checkvalves, valve guides, connection rods, bushes, crank shafts, cum lobes,cum journals, locker arms, valve springs, shims, lifters, rotating vanesof vane pumps, housings of vane pumps, timing chains, sprockets, chainguides (or chain guide shoes), chain tensioners (or chain tensionershoes) and the like. Specific examples of such sliding members of apower drive includes, for example, gears, chains, belts, rollerbearings, slide bearings, oil pumps etc. of automatic transmissions,continuously variable transmissions, manual transmissions, final drivegears, and the like.

The sliding member of the present invention is not limited to theabove-described members but can also be used in sliding conditions inthe presence of grease, biodiesel fuel, diesel fuel, gas-to-liquid (GTL)fuel, high-octane gasoline or the like.

The entire disclosure of Japanese Patent Application No. 2013-013290filed on Jan. 28, 2013 is incorporated herein by reference.

REFERENCE SIGNS LIST

-   1 Sliding member-   1′, 1″ Piston ring-   2 Base-   4 Hard chromium plated layer-   6 Hard carbon layer

The invention claimed is:
 1. A method for producing a sliding member,the sliding member comprising: a base, a chromium-based hard chromiumplated layer formed on a surface of the base, and a hard carbon layermainly composed of carbon element, formed on the hard chromium platedlayer, wherein a hydrogen concentration of the hard chromium platedlayer is from 10 to 140 mass ppm; comprising: heating the surface of thebase, on which the chromium-based hard chromium plated layer has beenformed, at a temperature of from 260° C. to less than 400° C., so thatthe hydrogen concentration of the hard chromium plated layer is from 10to 140 mass ppm, and thereafter forming the hard carbon layer mainlycomposed of carbon element on the hard chromium plated layer.
 2. Themethod according to claim 1, wherein a hydrogen concentration of thehard carbon layer is equal to or less than 40 at %.
 3. The methodaccording to claim 1, wherein a hydrogen concentration of an outermostlayer of the hard carbon layer is greater than 0 at % and less than orequal to 2 at %, or the outermost layer of the hard carbon layercontains no hydrogen.
 4. The method according to claim 1, wherein thesliding member is a piston ring, and the hard carbon layer is formed atleast on an outer peripheral surface of the piston ring.
 5. The methodaccording to claim 3, wherein the sliding member is a piston ring, andthe hard carbon layer is formed at least on an outer peripheral surfaceof the piston ring.
 6. The method according to claim 2, wherein ahydrogen concentration of an outermost layer of the hard carbon layer isgreater than 0 at % and less than or equal to 2 at %, or the outermostlayer of the hard carbon layer contains no hydrogen.
 7. The methodaccording to claim 2, wherein the sliding member is a piston ring, andthe hard carbon layer is formed at least on an outer peripheral surfaceof the piston ring.
 8. The method according to claim 6, wherein thesliding member is a piston ring, and the hard carbon layer is formed atleast on an outer peripheral surface of the piston ring.